1. Field of the Invention
The present invention relates to a printer for printing characters, graphics, photographic images or the like on a sheet. Specifically, the present invention relates to a printer that can print characters or the like on a sheet precisely while feeding the sheet accurately without decreasing print rate. “The printer” of the present invention is generic name of a device that can print characters or the like on a sheet. Therefore “the printer” of the present invention includes not only a printer with a single function, but also a copying device, a facsimile device, a multifunction device and the like being able to print characters or the like on a sheet. Characters, graphics, photographic images or the like to be print on a sheet are collectively referred to as “images” hereafter.
2. Description of the Related Art
The printer includes a feeding mechanism for feeding a sheet, and a print head for printing an image on the fed sheet. In the printer, a region provided with the print head is referred to as a printing region. The feeding mechanism feeds the sheet into the printing region, and feeds the sheet printed in the printing region out of the printing region. In the printing region, the print head prints an image while the print head is traveling across over the fed sheet. The direction in which the print head travels is referred to as main scan direction. The direction which crosses the main scan direction, and in which the sheet is fed is referred to as feeding direction or sub scan direction. An image to be printed on the sheet is divided into multiple lines extending in the main scan direction in a controller of the printer. Each time the print head travels across over the sheet, one line is printed on the sheet. The feeding mechanism feeds the sheet intermittently (or stepwise) each time one line is printed. To feed a sheet by one step is referred to as a line feed. An amount of time required for one line feed is referred to as a line-feed-time.
When the sheet is fed by one step, the print head again prints one line while traveling across over the sheet. In order to precisely print multiple lines on a sheet, namely, to print a high quality image, it is necessary to accurately feed the sheet intermittently.
The feeding mechanism often employs rollers. A sheet is fed by pressing rollers against the sheet, and rotating the rollers intermittently.
On the other hand, the print rate is preferably high. In order to feed a sheet intermittently at a high speed and accurately, rollers used to feed the sheet may be controlled with a feedback loop based on a rotation angle of the roller. Feedback control for rollers for feeding a sheet is disclosed in Japanese Patent Application Publication No. 2003-348878, for example.